The present invention relates to a turn-off circuit in use for gate turn-off thyristors (GTOs) widely used in, for example, large power inverters.
Conventional gate turn-off circuits for GTOs are arranged as shown in FIGS. 1 and 2. Basically, this type circuit is arranged so that a sharply rising pulse with a high peak value is formed by a pulse transformer, and a pulse such as a gate turn-off current is applied to the cathode gate path of the GTO. As shown in FIG. 1 a DC power source 1 is for turning off the GTO. A diode 2 is for preventing reverse flow of the current to the DC power source 1. A pulse transformer 3 has two segmented primary windings 4 and 5. An NPN transistor 6 is inserted between the battery 1 and the primary winding 5. As shown, the circuit components 1, 2, 4, 5 and 6 constitute a series circuit. A capacitor 8 is connected across a series connection of the primary winding 5 and the transistor 6. The primary windings 4 and 5 and a secondary winding 7 of the pulse transformer 3 are polarized as indicated by black dots which represent a high potential. A base control circuit 9 is connected to the NPN transistor 6 for controlling the base potential of the NPN transistor 6.
The base control circuit 9 is coupled with a control signal generating circuit 13. The control signal generating circuit 13 applies a turn-off signal to the base control circuit 9 and a gate control circuit 12 and a turn-on signal to a turn-on circuit 14. The secondary winding 7 and a thyristor 11, arranged in series fashion, are connected between the gate and the cathode of a GTO 10. The gate control circuit 12 is connected to the thyristor 11 for controlling the gate potential. The turn-on circuit 14, connected to the GTO 10, is driven by the control signal generating circuit 13 to turn the GTO 10 on.
The operation of the turn-off circuit shown in FIG. 1 will be given. For simplicity of explanation, it is assumed that the turn ratio of the primary windings 4 and 5 of the pulse transformer 3 is 1:1, and that the turn ratio of the primary windings and the secondary winding is n:1. In a stationary state, a voltage Ec across the capacitor 8 is higher than that of the DC power source 1, Ec&gt;E. This will be explained later. In this state, the diode 2 is reversely biased. The control signal generating circuit 13 has applied a turn-on signal to the turn-on circuit 14. The GTO 10, under control of the turn-on signal from the turn-on circuit 14, is in an ON state.
Under this condition, a turn-off signal with a predetermined pulse width is output from the control signal generating circuit 13. The signal is applied to the base control circuit 9 and the gate control circuit 12. Upon receipt of the turn-off signal, the base control circuit 9 and the gate control circuit 12 amplify the turn-off signal and apply a base-on signal and a gate-on signal to the NPN transistor 6 and the thyristor 11, respectively. The NPN transistor 6 and the thyristor 11 are turned on. At this time, the voltage Ec across the capacitor 8 is applied across the primary winding 5 of the pulse transformer 3 to induce a voltage Ec across the primary winding 4. Therefore, the potential at point A of the primary winding 4 connected to the diode 2 is 2 Ec. Accordingly, the diode 2 is reverse-biased, so that no current flows from the DC power source 1 to the pulse transformer 3. In this case, a voltage Ec/n appears in the secondary winding 7 of the pulse transformer 3.
The charge stored in the capacitor 8 is discharged at a time constant R1.times.C, where R1 is the equivalent resistance of the GTO 10 in the secondary winding when the equivalent resistance is converted in the primary winding 5, and C is the capacitance of the capacitor 8. At this time, an off-gate current, i.e. turn-off gate current, flows through the cathode-gate path in the secondary winding 7 of the pulse transformer 3. A waveform of the turn-off gate current can be shaped to have a sharp rising and a large peak value, by properly selecting the capacitance of the capacitor 8 and the inductance of the pulse transformer 3. As the discharge of the capacitor 8 progresses, the voltage across the capacitor 8 drops to E/2. At this time, the potential at point A is equal to the power source voltage E. When the potential at point B drops to below E/2 with further progression of the discharge of the capacitor 8, the diode 2 is forward-biased. Accordingly, the current from the DC power source 1 flows through a path containing the DC power source 1, the diode 2, the primary windings 4 and 5 and the NPN transistor 6. As a result, the voltage E/2 appears across the primary winding 4 of the pulse transformer 3. The reduced voltage E/2n is induced across the secondary winding 7. A relatively small current is supplied to the cathode-gate path of the GTO 10.
Under this condition, when the turn-off signal terminates and the transistor 6 is turned off, a voltage of inverse polarity is induced across the secondary winding 7 of the pulse transformer 3. The result is that the thyristor 11 is reverse-biased, the current flowing through the cathode-gate path of the GTO 10 disappears, and the GTO 10 is completely turned off. The above is the operation for turning off the GTO 10 in the turn-off circuit of FIG. 1. At the time of turn off, the current from the DC power source 1 flows into the capacitor 8 to charge the capacitor 8. The exciting energy of the pulse transformer 3 is stored in the capacitor 8 when the NPN transistor 6 is turned off. Therefore, the voltage across the capacitor 8 is approximately (Ec+2E) and the diode 2 is reverse-biased.
Another example of the prior art is shown in FIG. 2. For simplicity, only the portions of this example that differ from the previous one will be discussed. As shown, this example additionally contains two resistors 15 and 18 and two diodes 16 and 17. The resistor 15 and the diode 16 are connected in series between the positive side of the DC power source 1 and junction B between the primary windings 4 and 5. The resistor 15 is a charging resistor. The capacitor 8 is connected between a junction between the resistor 15 and diode 16 on one hand and the negative side of the power source 1 on the other hand. The diode 16 is for preventing current flow from the primary winding 4 to the capacitor 8. The diode 17 and the resistor 18 are connected across the primary windings 4 and 5, respectively. The combination of the diode 17 and the resistor 18 provides a circulating path to the exciting energy flow from the primary windings 4 and 5.
In the operation of FIG. 2, the assumptions made regarding the operation of FIG. 1 also apply. In a stationary state, the capacitor 8 is fed with a charge current from the DC power source 1 via the charge resistor 15. The voltage across the capacitor 8 is equal to the voltage of the DC power source 1.
Under this condition, a turn-off signal with a predetermined pulse width is generated by the control signal generating circuit 13. Upon receipt of the generated turn-off signal, the base control circuit 9 and the gate control circuit 12, respectively, produce and apply the base-on signal and the gate-on signal to the NPN transistor 6 and the thyristor 11. The NPN transistor 6 is turned on to allow the charge stored in the capacitor 8 to be discharged through the diode 16. The discharge current flows into the primary winding 5 to induce a voltage E/n in the secondary winding 7. At this time, the potential at the high potential point A of the primary winding 4 is 2E. Since 2E&gt;E, the diode 2 is reverse-biased to prevent the reverse current from flowing from the DC power source 1 to the pulse transformer 3. With progression of the discharging from the capacitor 8, the voltage across the capacitor 8 drops to E/2. At this time, the potential at the point A is equal to the power source voltage. When, the capacitor 8 is further discharged and the potential at point B drops to below E/2, the diode 2 is forward-biased. Then, the current flows through a route containing the DC power source 1, the diode 2, the primary windings 4 and 5 and the transistor 6. A voltage E/2n is induced across the secondary winding 7, so that a relatively small current flows through the cathode-gate path of the GTO 10.
Next, the turn-off signal terminates and the NPN transistor 6 is turned off. At this time, voltage is induced acorss the secondary winding 7 of the pulse transformer 3 with a polarity opposite to that of the previously induced voltage. Such induced voltage reverse-biases the thyristor 11 and turns off the thyristor 11 to cut off the current flowing through the cathode-gate of the GTO 10. In this way, the GTO 10 is completely turned off. When the NPN transistor 6 is turned off, the energy in the primary windings 4 and 5 of the pulse transformer 3 circulates through the diode 17 and the resistor 18, thereby preventing an overvoltage from being applied to the NPN transistor 6.
As described above, the turn-off circuit for the GTO requires a separate DC power source for the turn-off operation, as shown in FIGS. 1 and 2. This is undesirable particularly for an apparatus incorporating a plurality of GTOs such as a large power inverter. Such an apparatus uses a plurality of small DC power sources or a single large DC power source. This leads to increase in size, weight and cost. For turning off the large capacity or high frequency GTO, the power capacity of the DC power source is often insufficient. Further, a relatively large capacitor (not shown in FIGS. 1 and 2), just be connected in parallel to the DC power source. This fact makes the above problems critical.